1. Field of the Invention
The present invention relates to improved perpendicular magnetic recording media. In particular, the present invention relates to perpendicular recording media having a magnetic recording layer with improved magnetic properties, including improved perpendicular coercivity, to enhance the areal density of the perpendicular recording media.
2. Description of Related Art
Thin film magnetic recording media are typically composed of multiple layers, including one or more magnetic recording layers, disposed on a rigid substrate. Traditionally, the magnetic layer includes magnetic grains that are oriented longitudinally (i.e., in plane) with respect to the magnetic layer. The areal density of longitudinal magnetic recording disks has been increasing at a compounded growth rate of about 60% per year and areal densities as high as 100 Gbit/in2 have been demonstrated. However, with an increase in areal density, the volume of magnetic material within a single bit is reduced and the bit becomes highly susceptible to damage from thermal fluctuations, a phenomena referred to as a ‘superparamagnetic’ limit. This superparamagnetic limit places a constraint on increasing the areal density for longitudinal recording media beyond about 100 Gbit/in2.
To go beyond areal densities of 100 Gbit/in2, perpendicular magnetic recording media have been proposed. Perpendicular magnetic recording media include a magnetic recording layer having an easy magnetization axis that is perpendicular to the magnetic recording layer. Among the desirable properties for the magnetic recording layer used for perpendicular recording media are high perpendicular coercivity and anisotropy, a negative nucleation field and unity vertical squareness. Coercivity is a measure of the magnetization field that must be applied to reduce the remnant magnetization to zero, i.e., to reverse the direction of magnetization. A high perpendicular coercivity assures that the magnetic layer will have a high resistance to demagnetization by stray magnetic fields in the perpendicular direction. High perpendicular anisotropy means that the perpendicular coercivity is substantially higher than the horizontal coercivity. The nucleation field is the magnetic field that is necessary to begin to reduce the magnetization of the material to less than the saturation value. A material with a negative nucleation field has a highly stable remnant magnetization. Unity vertical squareness refers to a material property wherein the ratio of remnant to saturation magnetization (Mr/Ms) is one.
Cobalt-based alloys such as CoCr are typically utilized for the perpendicular magnetic layer. However, it is difficult to fabricate these and similar metal alloys such that all of the above magnetic properties are maintained in the magnetic layer.
It is known to use an underlayer disposed between the substrate and the magnetic layer in thin film recording media. For example, U.S. Pat. No. 4,632,883 by Howard et al. discloses the magnetic properties of a thin film alloy magnetic recording disk made with a CoCrX(X=Ta, Ti, Nb, Mo or W) magnetic layer and a β-Ta underlayer. The β-Ta underlayer has preferred [00.2] orientation and the vertical recording disk has an improved perpendicular coercivity (Hc⊥) of 1500 Oersted (Oe) and a horizontal coercivity (Hc∥) of 350 Oe.
U.S. Pat. No. 4,743,491 by Asada et al. discloses a perpendicular magnetic recording medium having an iron nitride (FexN) magnetic layer, an intermediate layer and an underlayer made of at least one element selected from the group consisting V, Ru, Zn, Os, Rh, Ir, Mo, W, Re, Pt, Nb, Ta, Sn, Al, Au, Ag, Ti, or electrically conductive nitrides and oxides of these elements.
There is a need for perpendicular recording media having improved magnetic properties to enable increased areal density. It would be advantageous to provide a perpendicular magnetic recording medium wherein the magnetic recording layer has high perpendicular coercivity, high perpendicular anisotropy, a negative nucleation field and unity vertical squareness.